The present invention relates to a method of producing a sintered fragmentation casing for explosives-charged warheads by applying powder metal technology. The present invention also includes various configurations of fragmentation casings produced in accordance with the said technology. A special feature of the fragmentation casings as claimed in the present invention is that their powder metal technology produced supporting main section or moulded part contains a large quantity of fragment bodies embedded at predetermined locations and distributed differently, and produced in a harder and heavier material than that used for the main mass of the moulded part. In this context the said fragment bodies are preferably comprised of heavy metal balls.
By powder metal technology is meant here that the single-piece supporting main section or moulded part is completely or partially formed by a suitable powder metal that is compressed until it assumes the desired form and is sintered together to form a homogenous metal.
Two different methods of producing homogenous metal bodies using powder metallurgy technology are well known. One of the said methods is designated in everyday language as HIP-ing or hot isostatic pressing which means that the basic powder material being used is isostatic compressed at the same time as it is sintered to form a homogenous metal. The other method is designated SIP-ing which means that the powder material is first cold isostatic compressed until the desired density is achieved, after which the compressed powder granules are sintered in a separate process until a homogenous metal is formed.
Both of these general methods can be utilised within the basic concept of the present invention.
By the designation heavy metal is meant here primarily high density Wolfram alloys. Depleted uranium has also been used in similar circumstances but it is still regarded with doubts regarding its effect on health during handling prior to use as well as any radioactive fallout after use.
When combating airborne targets such as aircraft and various types of missiles using barrel-fired projectiles or own missiles, as a rule it cannot be counted on that a direct hit on the target will be achieved and instead a near-miss must suffice and that the explosive charge-loaded warhead can be detonated as close to the target as possible. For this to be enabled the said warhead must be provided with a proximity fuze or equivalent that controls its detonation until the optimal point in time for combating the target with pressure and fragments. In most cases the greatest effect in the target from the said type of near-miss is achieved when the explosive charge is enclosed in a fragmentation jacket comprising a large number of pre-formed fragment bodies. Heavy metal bodies are now assumed to be the best technical and most economic fragment bodies as they have a high level of density and when they are enclosed in a fragmentation jacket they also create large quantities of fragments. The said heavy metal balls that are projected at high velocity by the detonation of the explosive generate good penetration even in semi-hard targets and in addition their size and consequently their dispersion pattern are predetermined. On the other hand it is more difficult to determine exactly how an originally homogenous fragmentation jacket for an explosive charged projectile will disintegrate when subjected to the detonation of an explosive charge and consequently the fragment dispersion pattern thus formed will be difficult to determine and partially at random. Therefore the intention was to provide air defense explosive-charged projectiles with a fragmentation jacket containing a large quantity of heavy metal balls that when the explosive is detonated it will eject a swarm of the said heavy metal balls in the direction of the target. However, to produce such a fragmentation jacket is not the easiest of tasks because the object is to have the greatest possible number of heavy metal balls penetrate the target and therefore the form of the fragmentation jacket is a critical factor in this context. Even in relatively simple forms this type of fragmentation jacket is relatively problematic to manufacture using the technologies currently available.
In this context U.S. Pat. No. 3,815,504 describes a method of producing fragmentation jackets for use in artillery shells where heavy metal balls are filled in between an inner and an outer tubular casings until the space between them is completely filled after which the inner tubular casing is subjected to high inner pressure either via a slightly conical “dolly” device or an inner detonation which secures the heavy metal balls by means of deformation of the inner tubular casing.
The said method of producing fragmentation jackets however, has the disadvantage of leaving a gap between the heavy metal balls which at an early stage of the detonation phase of the explosive contained in the complete shell causes pressure leakage between the said heavy metal balls thus exerting a lower velocity on them than would have been the case had they been completely encased by a moulded part.
U.S. Pat. No. 4,503,776 further describes a fragmentation jacket comprising projectile-formed fragment bodies that are provided with a rear free opening that is used partly to fix the said fragment bodies in position in a fixture while the said fragment bodies are moulded in a base material, and partly for filling with incendiary material or equivalent after the moulding process is completed and the fixture has been removed. The moulding material used is cast iron and the said fixture can be of a ceramic material that can be either left in place or be removed when the moulding base material has set. The most immediate problem with this method would appear primarily to be the risk of porosity in the moulded material.
Finally, U.S. Pat. No. 4,129,061 describes a prefragmented shell having an outer casing produced using powder metallurgy technology. In this variant a compact layer of heavy metal balls is arranged around a single-piece body and thereafter the said compact layer of heavy metal balls is covered with powder metal that is then compacted and sintered together after which the centre body bored out to receive the explosive charge and the sintered powder jacket is finish-machined to the intended shape of the shell. However, the said patent does not disclose how the heavy metal balls are retained in their positions until the powder metal is introduced and compacted to form a single unit. Moreover, the said method requires considerable subsequent work and creates the risk of irregular powder density in critical areas.
Several years ago we made several attempts to produce shells provided with a prefragmented casing by applying powder metallurgy technology but the results were not completely satisfactory. Even though current conventional powder metallurgy technology is used to produce a large variety of different products there is a particular problem involved when producing prefragmented casings, namely the said casings shall contain such a large quantity of separately produced heavy metal balls from the very start. That is to say it is the material between the heavy metal balls that holds them together and gives the prefragmented casing its outer form that is to be created by powder metallurgy technology and inside the said single-unit casing or moulded part the heavy metal balls shall be embedded.
This is to say that a prefragmented shell casing containing embedded heavy metal balls comprises two different materials of which the heavy metal balls are already produced completely prior to formation of the single-unit casing and compaction by the powder metal that is then sintered together to form a single homogenous unit. The greatest difficulties with manufacturing prefragmented shell casings by applying powder metallurgy technology is that the materials to be included will have completely coefficients of expansion while the sintering phase involves the entire pre-formed body must be heated to the sintering temperature of the powder component. In previous attempts to produce prefragmented casings by applying powder metallurgy technology the frequency of shrinkage cracks in the casings was so high that as far as we are aware they never appeared on the market.
Previously tested techniques in this field are described in Swedish Patent SE 450294 (=U.S. Pat. No. 4,644,867) represented in the form of powder metallurgy technology produced prefragmented shells the casings of which were produced by means of completely pre-formed heavy metal balls embedded in powder metal that are then subjected to high temperature and high pressure from all directions to form a tightly compacted casing. Even if this patent, which is our own, does not state clearly how we were able to retain the heavy metal balls in their correct positions in the metal powder jacket, at that period of time we utilised a technique where we first attached the pre-formed heavy metal balls to a single-piece prefragmented casing which we then surrounded with powder steel which was then compacted under high pressure and sintered together to form a single uniform material. The problem using this technique was that the heavy metal balls formed a single inter-connected layer having completely different shrinkage properties than the surrounding powder metal technology produced material. Consequently, the frequency of cracks in the powder metal technology produced fragmentation jacket was too high for the production method to be utilised for mass production.
Unless we are very much mistaken the inventors responsible for U.S. Pat. No. 4,129,061 must have experienced a similar problem only more extensive as in the sintering phase their product contained compacted powder metal, a fragmentation jacket comprised of tightly packed heavy metal balls and an inner “dolly” that had a very large volume compared with the rest of the material.